| Attributs | Valeurs |
|---|
| type
| |
| Is Part Of
| |
| Subject
| |
| Title
| - Asymmetric Aldol Additions: Use of Titanium Tetrachloride and(−)-Sparteine for the Soft Enolization of N-Acyl Oxazolidinones,Oxazolidinethiones, and Thiazolidinethiones
|
| has manifestation of work
| |
| related by
| |
| Author
| |
| Abstract
| - Asymmetric aldol additions using chlorotitanium enolates of N-acyloxazolidinone, oxazolidinethione,and thiazolidinethione propionates proceed with high diastereoselectivity for the Evans or non-Evans syn product depending on the nature and amount of the base used. With 1 equiv of titaniumtetrachloride and 2 equiv of (−)-sparteine as the base or 1 equiv of (−)-sparteine and 1 equiv ofN-methyl-2-pyrrolidinone, selectivities of 97:3 to >99:1 were obtained for the Evans syn aldolproducts using N-propionyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones. The non-Evans syn aldol adducts are available with the oxazolidinethione and thiazolidinethiones by alteringthe Lewis acid/amine base ratios. The change in facial selectivity in the aldol additions is proposedto be a result of switching of mechanistic pathways between chelated and nonchelated transitionstates. The auxiliaries can be reductively removed or cleaved by nucleophilic acyl substitution.Iterative aldol sequences with high diastereoselectivity can also be accomplished.
|
| article type
| |
| is part of this journal
| |
